1. Field of the Invention
The present invention relates to an encoding device, an encoding method, and a program product including an encoding method.
2. Description of the Related Art
Conventionally, various researches have been done for audio coding technology for compressing/decompressing audio signals as sound sources for voice, music, and so on. For example, various researches are directed to schemes for encoding audio signals through conversion into frequency domain.
For example, such audio coding technology will be found in Advanced Audio Coding (ACC) method, High Efficiency-Advanced Audio Coding (HE-AAC) method. The AAC and the HE-AAC methods are ones of the ISO/IEC MPEG-2/4 audio standards and are widely used in a digital broadcasting, such as the digital terrestrial, the BS digital, and the Communication Satellite, and one segment broadcastings in Japan.
In such audio coding technology, a conventional encoding device for implementing the audio coding technology converts an audio signal into frequency spectra by Modified Discrete Cosine Transform (MDCT) conversion, quantizes the frequency spectra, and then performs encoding.
The conventional encoding device quantizes the frequency spectra by utilizing auditory masking properties. Specifically, the conventional encoding device quantizes only sound that can be heard by human auditory perception. In the quantization, a masking threshold as a threshold is used to determine components of sound that cannot be acoustically heard, namely the threshold for whether sound can be heard or not.
For example, the conventional encoding device performs psychoacoustic analysis, which is a scheme for analyzing whether or sound is acoustically heard or not, with respect to an audio signal (a sound source to be encoded). Then masking thresholds are determined for each frequency. Thereafter, for each band having a predetermined frequency width, the conventional encoding device determines an error limit. The error limit is an allowable error power that is allowed during quantization, based on the determined masking threshold. Then, using the allowable error power, the conventional encoding device quantizes only frequency spectra as a sound source that is acoustically heard.
Japanese Laid-open Patent Publication No. 2006-18023, pages 5 to 11 and FIG. 1, discloses a scheme for adjusting a masking threshold, Japanese Laid-open Patent Publication No. 2001-7704, pages 5 to 9 and FIG. 1, discloses a scheme for improving efficiency during encoding for reducing the usage amount of bits used during encoding. In addition, Japanese Laid-open Patent Publication No. 7-202823, pages 3-5 and FIG. 1, and Japanese Laid-open Patent Publication No. 7-295594, pages 2 to 3 and FIG. 1 disclose schemes for specifying the amount of bit distribution.
Meanwhile, the above-described conventional technologies have a problem in that the sound quality deteriorates during encoding of a tonal high audio signal.
In more detail, since the conventional encoding device cannot reliably quantize frequency spectra adjacent to the peak during encoding of a tonal audio signal, and the device cannot satisfactorily perform encoding while maintaining a sufficient sound quality.
The Japanese Laid-open Patent Publications described above do not disclose a scheme for reliably quantizing frequency spectra adjacent to the peak and cannot sufficiently improve the sound quality during encoding of a tonal audio signal.